Recently, a thermoelectric power-generating technology for which the system is simple and can be down-sized has been specifically noted as a power recovery technology for unharnessed waste heat energy that is generated from fossil fuel resources and others used in buildings, factories, etc. However, thermoelectric power generation is, in general, poorly efficient in power generation, and therefore, studies and developments are being actively made for improving power generation efficiency in various companies and research institutes. For improving power generation efficiency, it is indispensable to enhance the efficiency of thermoelectric conversion materials, and for realizing it, it is desired to develop materials having a high electric conductivity comparable to that of metals and having a low thermal conductivity comparable to that of glass.
A thermoelectric conversion characteristic can be evaluated by a thermoelectric performance index Z (Z=σS2/λ). Here, S means a Seebeck coefficient, a means an electric conductivity (reciprocal of resistivity), and λ means a thermal conductivity. Increasing the value of the thermoelectric performance index Z improves the power generation efficiency, and for enhancing the efficiency in power generation, it is important to find out a thermoelectric conversion material having a large Seebeck coefficient and a large electric conductivity σ, and having a small thermal conductivity λ.
In general, the thermal conductivity λ and the electric conductivity σ of a solid substance can be planned using the density of the material and the carrier concentration as parameters; however, the two physical properties are not independent of each other owing to the Wiedemann-Franz law but coordinate closely with each other, and therefore, in fact, it has heretofore been impossible to significantly improve the thermoelectric performance index. Given the situation, PTL 1 proposes a thermoelectric conversion material that has been prepared by introducing a large number of supermicroscopic pores into the inside of a semiconductor material as dispersed therein at intervals equal to or smaller than the mean free path of electrons and phonons, so as to make the material porous to thereby reduce the thermal conductivity and increase the Seebeck coefficient thereof. According to Examples in PTL 1, the thermal conductivity reduced but the electric conductivity also reduced (the resistivity greatly increased), and the non-dimensional thermoelectric performance index ZT (at T, absolute temperature, 300 K) merely increased from 0.017 to 0.156 through porous structure formation, and the situation is that the absolute value is far from the index value for practical use, ZT≧1.
In PTL 2, there is given an investigation for forming a micro-cylinder structure, in which, on a coating film formed of a coating liquid that contains a general-purpose polymer such as polystyrene or the like and a hydrophobic organic solvent such as methylene chloride or the like, a nano- or micro-scale water vapor-containing gas, of which the dew point is controlled to be higher than the temperature of the coating film, is sprayed and condensed thereon, and the vaporization of water condensed in the hydrophobic organic solvent is stepwise repeated. In this, however, it is difficult to control the condition, and in particular, the distance between the pores is fluctuated, the pore area proportion is small, and therefore the structure is not suitable for a porous structure for use for a thermoelectric conversion material.